The present invention relates to a sintered aluminum nitride suitable for insulating substrates for integrated circuits, etc., and more particularly to sintered aluminum nitride having a high thermal conductivity, a high electrical resistivity, and a low coefficient of thermal expansion.
The semiconductor industry has made a remarkable progress in recent years and a number of circuit constituents such as semiconductor chips have been formed in increasingly higher density on an insulating substrate used in large scale integrated circuit and the like. Demands for devices having greater capacity with smaller size have also become keener and insulating substrates having a high thermal conductivity have been in demand.
As materials for such an insulating substrate, there has conventionally been employed sintered alumina. Since the sintered alumina does not have a satisfactory thermal conductivity and development of an insulating substrate having a higher thermal conductivity has been requested. The insulating substrate for such an application must satisfy the following requirements.
(1) high electrically insulating property, PA1 (2) high thermal conductivity, PA1 (3) its coefficient of thermal expansion is approximate to that of silicon, PA1 (4) high mechanical strength, and PA1 (5) low dielectric constant.
Sintered silicon carbide has been proposed to satisfy the foregoing requirements, but still has a problem in obtaining a high density sintered silicon carbide with a high electrical resistivity, and various improvements have been therefore proposed. That is, various sintering aids have been employed in order to obtain a high density sintered silicon carbide. U.S. Pat. No. 4,172,109, for example, discloses a sintered silicon carbide containing beryllium as a sintering aid. This prior art relates to a high strength material obtained by sintering powder of silicon carbide which contains 0.5 to 5 wt. % of excessive carbon. However, the sintered silicon carbide thus formed has small electric resistivity and cannot be used as an electrically insulating material.
On the other hand, U.S. patent application Ser. No. 203,554 filed Nov. 5, 1980 now U.S. Pat. No. 4,370,421 discloses sintered silicon carbide containing 0.1-3.5 wt. % beryllium, which has a thermal conductivity of at least 0.4 cal/cm.sec..degree.C. at 25.degree. C., an electrical resistivity of at least 10.sup.7 ohm.cm at 25.degree. C. and a coefficient of thermal expansion of 3.3-4.times.10.sup.-6 /.degree.C. at 25.degree. C. to 300.degree. C. However, in that case, the firing temperature is as high as 1,850.degree.-2,500.degree. C., and a new material, which can be fired economically at a lower temperature and has a higher electrical resistance, has been in demand.
As a substitute for the sintered silicon carbide, sintered aluminum nitride has been regarded as important, because sintered aluminum nitride has a coefficient of about 5.times.10.sup.-6 /.degree.C., which is smaller than that of sintered alumina, about 7.times.10.sup.-6 /.degree.C., and is nearly as low as that of silicon, about 3.3.times.10.sup.-6 /.degree.C., a flexural strength of about 50 kg/mm.sup.2, which is much higher than that of sintered alumina, about 20 kg/mm.sup.2, and also has a good electrically insulating property.
The well known methods for preparing sintered aluminum nitride include (1) a reaction sintering method, (2) normal pressure sintering method, and (3) hot press sintering method [Journal of Material Science 4 (1969) 1045-1050; Journal of The American Ceramic Society-Discussions and Notes, 57 No. 9, 411-412]. The reaction sintering method is to sinter a metallic aluminum molding while subjecting it to a nitriding reaction in a nitrogen gas atmosphere, where the rate of nitriding reaction is dependent upon diffusion of nitrogen gas, so that, in the case of a thick molding, unreacted aluminum remains at the center part and the sintered molding is porous, so that it cannot be practically used as an electrically insulating material.
The normal pressure sintering method is to sinter a molding of a mixture of aluminum nitride powder and a powdery additive of yttrium oxide and a rare earth element oxide, or yttrium oxide and silicon dioxide, nickel and/or calcium oxide.
The hot press sintering method is to sinter a molding of a mixture of aluminum nitride powder and a powdery additive of aluminum oxide, or yttrium oxide and silicon dioxide, or the like with heating under pressure. According to the known normal pressure sintering method and hot press sintering method, compact sintered aluminum nitride with a high mechanical strength, a high electrically insulating property and a low coefficient of thermal expansion can be obtained. However, the sintered aluminum nitride has a low thermal conductivity, usually 0.07 cal/cm.sec..degree.C. and at most 0.1 cal/cm.sec..degree.C. at room temperature. Thus, sintered aluminum nitride having a higher coefficient of thermal conductivity, if available, will be very useful as an insulating substrate material for large scale integrated circuits etc.